Microalgal responses to physico-chemical variability in the small temporarily open/closed Seteni Estuary,South Africa

The Seteni Estuary is a small temporarily open/closed estuary (TOCE) in South Africa under the influence of agricultural practices. While the general significance of microalgae to estuarine production is widely recognised, the factors regulating microalgal biomass in these heterogeneous systems are less well understood, particularly when man-induced pressures are superimposed on natural variability. This study investigated microalgal responses to physico-chemical variability in the nutrient-enriched Seteni Estuary in 2008–2009. Microphytobenthic biomass ranged from 1.1 to 91.7 mg Chl a m^-2, while phytoplankton biomass varied between 0.22 and 18 mg Chl a m^-3. Despite the high dissolved inorganic nitrogen (DIN) concentrations recorded, salinity and rainfall were identified as the main environmental drivers, highlighting the importance of allochthonous inputs. While this system appears to function as a typical TOCE, the relatively low microalgal biomass recorded may be an artifact of several factors such as phosphorus limitation, herbicide treatment, etc., acting singly or together to depress biomass levels. However, should the balance be upset, the most probable result would be a dramatic increase in microalgal biomass, to the point where harmful algal blooms may ensue. The system therefore needs to be closely monitored to prevent further degradation.     

The seed banks of two temporarily open/closed estuaries in South Africa

The seed banks of two temporarily open/closed estuaries in South Africa were quantified in this study. Charophyte öospores represented almost 72% of the sexual propagules in the sediment with a mean öospore density of 31,306 öospores m⁻². This was followed by the seeds of the intertidal salt marsh plant Sarcocornia perennis (18%) (7929 seed m⁻²) and the submerged angiosperm Ruppia cirrhosa (7%) (2852 seeds m⁻²). The remaining 3% was made up of a mixture of species such as Salicornia meyeriana, Sporobolus virginicus, Stukenia pectinata, Bolboschoenus maritimus and terrestrial species. Although seed density did not differ significantly with depth, seeds still occurred at 20 cm depth providing a regeneration source in the event of sediment disturbance. Three salinity (0, 17 and 35 PSU) and moisture treatments (exposed, waterlogged and submerged) were applied to collected sediment to determine how fast species would germinate. S. perennis germinated after 3 d to a maximum of 82%. Submerged species began to germinate only after 18 d (Chara vulgaris and R. cirrhosa) and had low germination percentages of between 11 and 15% after 91 d. Results from this study indicate that in the event of unpredictable disturbance events such as water level fluctuations, large sediment seed reserves would ensure habitat persistence.     

A comparison of the macrobenthic faunas of permanently open and temporarily open/closed South African estuaries

Thirteen estuaries in the Eastern Cape Province, South Africa, were broadly categorised according to size and salinity distribution and were assigned to one of the following categories: permanently open estuaries having a strong salinity gradient between mouth and upper estuary, freshwater-deprived permanently open estuaries, medium-sized temporarily open/closed estuaries, and small, temporarily open/closed estuaries. The macrobenthos collected during surveys was then compared in terms of the following parameters: species composition, salinity, sediment mud content, density of macrobenthic animals, Hill's N0 (species richness), and Hill's N1 (diversity). Mud content was found to be the most important environmental variable responsible for biotic patterns found, and sites were consequently assigned to either a sand zone fauna, or a mud zone fauna. Both types of fauna are present in all estuaries sampled, with upper sites of river dominated estuaries having an additional oligohaline fauna, and freshwater-deprived estuaries providing habitat for many marine species. Small, temporarily open/closed estuaries have the highest macrobenthic density, whereas N0 and N1 are highest in freshwater-deprived permanently open systems. River-dominated permanently open estuaries tend to have lower macrobenthic densities, species richness, and diversities compared to estuaries in the other categories. No seasonal differences in these ecological indices were found within any of the estuarine categories.     

Fish communities in temporarily open/closed estuaries from the warm- and cool-temperate regions of South Africa: A review

The majority of estuaries along the coastline of southern Africa are termed temporarily open/closed estuaries (TOCEs) and are closed off from the sea for varying periods by a sandbar which forms at the mouth. It is therefore important to understand the processes occurring within TOCEs and their importance to fishes in order to make sound management recommendations. Estuaries along the coast of South Africa and their associated fish assemblages are biogeographically distinct and occur in either a subtropical, warm-temperate or cool-temperate zone. There are 125 TOCEs found within the cool-temperate and warm-temperate zones. Most fish species found in TOCEs are the juveniles of marine taxa that breed at sea. Permanently open estuaries generally have a higher diversity of species than TOCEs, but TOCEs still provide important nursery areas for many marine species and numerically often have a higher proportion of estuarine resident species. Important taxa in terms of abundance and biomass in warm-temperate TOCEs include the sparids Rhabdosargus holubi and Lithognathus lithognathus, several mugilid species, estuarine residents (particularly Gilchristella aestuaria and Atherina breviceps) and the freshwater cichlid Oreochromis mossambicus. The diversity of fishes in cool-temperate TOCEs is low when compared with warm-temperate systems and Liza richardsonii tends to dominate catches by number and mass in most systems. Several species recorded in TOCEs show clear longitudinal distribution trends. For example Atherina breviceps is generally more abundant in the lower reaches of estuaries. Mouth state, particularly the frequency, timing and duration of mouth opening plays a key role in determining species richness, composition, diversity and abundance in TOCEs. Mouth state is directly linked to freshwater input. Reduced river inflow leads to prolonged mouth closure and shorter open phases, which inhibits immigration and emigration of marine fish species between estuaries and the sea. Understanding of the effects of various processes occurring within these systems, particularly variation in freshwater input, on the biota of these important systems facilitates the development of informed management recommendations.     

Responses of primary producers to mouth closure in the temporarily open/closed Great Brak Estuary in the warm-temperate region of South Africa

Low river inflow conditions during 2009/2010 resulted in the mouth of the Great Brak Estuary remaining closed for almost two years. The low water level in Wolwedans Dam resulted in no annual environmental flow releases being made, causing mouth closure. The response of primary producers to this prolonged period of mouth closure was investigated in 2010/2011. Urban and agricultural development in the river catchment and along the estuary banks had increased the nutrient inputs into the estuary. Mouth closure, combined with elevated nutrient concentrations, increased the growth of both macroalgae and microalgae, but little change was observed in the submerged macrophytes. Macroalgal mats covered large areas of open-water surface, smothering the salt marsh and causing a decrease in its cover. These results have important implications for the management of temporarily open/closed estuaries, as increased development, freshwater abstraction and reduced river flow will result in prolonged periods of closure and reduced tidal exchange, which is likely to lead to eutrophication.     

Subtidal macrozoobenthic community resilience to change following natural mouth breaching of the temporarily open/closed East Kleinemonde Estuary,South Africa

The East Kleinemonde Estuary, on the south-east coast of South Africa, typically opens for only short periods, usually <10 days. During a nine-month study in 2006, marine influence persisted for a near-continuous period of three months. This provided an opportunity to explore potential changes in the structure and pattern of the subtidal estuarine macrozoobenthic community. Salinity changes were minimal after mouth opening, decreasing by c. 4 and 8 in the lower and upper estuary, respectively. Salinity decrease was due to river dominance over tidal influence. Two biotic assemblages, structured by sediment characteristics, typified the community throughout the study: a sand-associated group near the mouth and a mud-associated group upstream. Amphipods were proportionally the most abundant group within each assemblage and on each sampling occasion. At the species level, high variability typified the community, with no trends or patterns observed. However, species richness declined from 27–30 to 23 after mouth opening. The number of species returned to former levels at the time of the final survey and just before final mouth closure. An analysis of the subtidal macrozoobenthos in the East Kleinemonde and other temporarily open/closed estuaries (TOCEs) of similar size in the bioregion indicated that communities were typically composed of relatively few, broadly tolerant euryhaline species with no species unique to TOCEs.     

Feeding dynamics of Palaemon peringueyi (Decapoda,Caridea) in the temporarily open/closed Kasouga Estuary,South Africa

The feeding ecology of the caridean shrimp Palaemon peringueyi in the temporarily open/closed Kasouga Estuary, Eastern Cape, was examined during summer and winter employing stomach fullness index (SFI) analysis. Mean SFI values for male and female shrimp during summer ranged from 0.85% to 2.36% and from 0.56% and 2.61% body dry weight, respectively. During winter, SFI values for males and females were lower, and varied between 0.23% and 1.56% and 0.36% and 1.84% body dry weight, respectively. There were no significant differences in the SFI values between males and females during the two seasons. A peak in the feeding activity for both sexes was observed during the daytime during both summer and winter. For both males and females, non-linear regression analysis best explained the change in SFI over time. The gut evacuation rate constants (k) of males and females during summer were estimated at 0.43 h^?1 and 0.30 h^?1, respectively. These values correspond to a daily food intake equivalent to 16.2% body dwt for males and 11.1% body dwt for females. During winter, the gut evacuation rates of the shrimp were estimated at 0.35 h^?1 for males and 0.28 h^?1 for females. The daily ration for adult male and female shrimp during winter was estimated at 6.1% and 6.8% body dwt, respectively. The lower estimates obtained during winter appear largely to reflect the influence of the reduced water temperatures on the metabolic activities of the shrimp.     

Bacterial responses to natural organic inputs in a marine subAntarctic area

Quantitative investigations of bacteriological and organic matter parameters were conducted regularly at least once a week for a year at three subAntarctic benthic stations. The different bacteriological methods used (direct counts, viable counts and heterotrophic potentials) gave complementary results. Despite substantial differences in organic matter content of the different sediments, the bacterial communities were of similar density. Seasonal variations observed in the studied area were regular enough to allow a detailed analysis. A general scheme describing the reactions of the bacterial microflora to the different periods of organic input is discussed. This model, which includes three parts (lag phase, growing phase and return to the initial state), allows the interpretation of the bacterial responses observed during the benthic surveys.     

An assessment of the size structure, distribution and behaviour of fish populations within a temporarily closed estuary using dual frequency identification sonar (DIDSON)

The fish populations within the East Kleinemonde Estuary, South Africa, were studied using Dual frequency IDentification SONar (DIDSON) during March 2010 which allowed for direct comparisons of the abundance, distribution and behaviour of a wide size range of fishes along the length of the system. Small schooling fishes <100 mm in total length (L(T) ) were most abundant while larger size classes 100-300 and 300-500 mm were less abundant, but evenly distributed longitudinally within the estuary. Fishes <100 mm were most abundant within the mouth region which may reflect the feeding ecology of species that fall into these size classes and the effects of higher turbidities further up the estuary. Large fishes >500 mm were similarly most abundant near the mouth, and fishes of this size being mostly piscivorous, probably showed these distribution patterns due to the location of smaller prey fishes. Differential behaviour was observed among the longitudinal sections, with small fishes forming schools in areas of low turbidity and particularly in the presence of large predatory fishes. Results from this study present a snap-shot of the composition, distribution and behaviour of the ichthyofauna within a small temporarily closed estuary, and also show that abiotic factors and interactions of various size classes can influence their distribution. There is significant potential for DIDSON in future ecological research within these systems.     

Convergent responses of nitrogen and phosphorus resorption to nitrogen inputs in a semiarid grassland

Human activities have significantly altered nitrogen (N) availability in most terrestrial ecosystems, with consequences for community composition and ecosystem functioning. Although studies of how changes in N availability affect biodiversity and community composition are relatively common, much less remains known about the effects of N inputs on the coupled biogeochemical cycling of N and phosphorus (P), and still fewer data exist regarding how increased N inputs affect the internal cycling of these two elements in plants. Nutrient resorption is an important driver of plant nutrient economies and of the quality of litter plants produce. Accordingly, resorption patterns have marked ecological implications for plant population and community fitness, as well as for ecosystem nutrient cycling. In a semiarid grassland in northern China, we studied the effects of a wide range of N inputs on foliar nutrient resorption of two dominant grasses, Leymus chinensis and Stipa grandis. After 4 years of treatments, N and P availability in soil and N and P concentrations in green and senesced grass leaves increased with increasing rates of N addition. Foliar N and P resorption significantly decreased along the N addition gradient, implying a resorption‐mediated, positive plant–soil feedback induced by N inputs. Furthermore, N : P resorption ratios were negatively correlated with the rates of N addition, indicating the sensitivity of plant N and P stoichiometry to N inputs. Taken together, the results demonstrate that N additions accelerate ecosystem uptake and turnover of both N and P in the temperate steppe and that N and P cycles are coupled in dynamic ways. The convergence of N and P resorption in response to N inputs emphasizes the importance of nutrient resorption as a pathway by which plants and ecosystems adjust in the face of increasing N availability.     

Enzymatic responses to metal exposures in a freshwater fish Oreochromis niloticus

Freshwater fish Oreochromis niloticus were exposed to 5, 10 and 20 microM concentrations of Cd, Cu, Zn and Pb for 14 days and responses of several enzymes were determined subsequently. Liver catalase (CAT) activity was influenced by Cd and Pb exposures, while it was inhibited by Zn exposure. Copper, on the other hand, did not cause significant changes in CAT activity. Liver alkaline phosphatase (AP) activity was first stimulated at lowest (5 microM) exposure concentration, while there were significant inhibitions at higher (10 microM) exposure concentration. At the highest (20 microM) exposure concentration, AP activity was compensated coming to the control level, except Pb exposure. Intestine and serum AP activities were stimulated by all Zn exposures and 10 microM Cu exposure, while other exposures did not cause significant changes on AP activity. Na,K-ATPase activity in the gill and intestine was inhibited by all the metal exposures, except 20 microM Pb exposure that resulted in an increase in the activity in the gill. Similarly, muscle Ca-ATPase activity was inhibited by all the metal exposures, except Cu exposures. This study indicated that enzymatic systems may be used as a sensitive bioindicator of metal contamination in aquatic systems. Nevertheless, the results also stress the importance of addressing the sensitivity of these enzymes when they are used as a bioindicator for metal contamination, because the responses of the enzymes varied considerably according to metal types and concentrations.     

Behavioural responses of freshwater phytoplanktonic flagellates to a temperature gradient

Photoresponses of motile phytoplanktonic flagellates have been widely studied, whereas the behavioural responses of these organisms to temperature, and their potential ecological consequences, have rarely been considered. This study investigated the population responses and individual swimming trajectories of five phylogenetically contrasting species of freshwater flagellates exposed to a gradient of temperature in micro-scale preference chambers. Population responses demonstrated a species-dependent diversity in thermoresponsive behaviour. Across a gradient of 9.8 to 15.1°;C which simulated the typical range of temperature across a thermocline in a temperate monomictic lake, Ceratium furcoides, Chlamydomonas moewusii, Dinobryon sertularia and Plagioselmis nannoplanctica all preferred the highest temperatures. In contrast, Euglena gracilis preferred the lowest temperature. Analysis of the swimming behaviour of individual cells confirmed preferences and demonstrated that, in all species, a combination of tactic and kinetic reactions was responsible for these accumulations. For the first time, controlled positive and negative thermotaxes towards a temperature preference were identified. This thermotactic orientation, in conjunction with a reduction in directionality of response in the preference zone and with ortho- and klino-thermokinesis, enabled cells to maintain position at preferred temperatures. Specifically, in all species apart from P. nannoplanctica, significant ortho-kinetic increases in swimming speed permitted rapid movement of cells away from unfavourable conditions, while a reduction in speed and a klino-kinetic increase in rate of turning (in all five species) maintained position within favoured temperatures. This ability to detect, orientate and accumulate within a temperature gradient may be triggered by physiological processes and presents ecological advantages. Behavioural response to temperature may optimize growth, influence the spatial and temporal distribution of flagellates, particularly the diel position of cells during migration, and contribute to the delineation of niche separation.     

The importance of runoff to DDT and PCB inputs to the Sado estuary and Ria Formosa

PCB congeners, DDT and metabolites were determined in suspended sediments and in the oysterCrassostrea angulata from the upper Sado estuary over two years (1987/88 and 1990) and from Ria Formosa during 1990. Levels of DDT concentrations in suspended sediments collected in both estuarine areas, either in 1988 or 1990, increased in winter. Otherwise, PCB concentrations varied in an erratic way. A DDT enrichment was also recorded inC. angulata in winter and an increase of only PCB concentration levels in oysters of the upper Sado estuary was registered at the same period. These results suggest that, in the absence of important direct sources of PCB and DDT, runoff appears to be the major source of those compounds in both the study areas. Levels of concentrations of these pollutants in oysters reflect either their increase in the environment or changes in particle dynamics in the two estuarine areas.     

Molecular dynamics simulation of tropomyosin bound to actins/myosin in the closed and open states

Tropomyosin (Tpm) is a dimeric coiled-coil protein that binds to filamentous actin, and regulates actin-myosin interaction by moving between three positions corresponding to the blocked, closed, and open states. To elucidate how Tpm undergoes transitions between these functional states, we have built structural models and conducted extensive molecular dynamics simulations of the Tpm-actins/myosin complex in the closed and open states (total simulation time >1.4 μs). Based on the simulation trajectories, we have analyzed the dynamics and energetics of a truncated Tpm interacting with actins/myosin under the physiological conditions. Our simulations have shown distinct dynamics of four Tpm periods (P3-P6), featuring pronounced biased fluctuations of P4 and P5 toward the open position in the closed state, which is consistent with a conformational selection mechanism for Tpm-regulated myosin binding. Additionally, we have identified key residues of Tpm specifically binding to actins/myosin in the closed and open state. Some of them were validated as functionally important in comparison with past functional/clinical studies, and the rest will make promising targets for future mutational experiments.     

Stomatal and photosynthetic responses to variable sunlight

Most plants experience many fluctuations in sunlight from full sun to shade throughout the day. Under these conditions, stomatal and photosynthetic responses vary dramatically among species depending on water status and growth form. Many herbaceous, fast-growing species rapidly reduce stomatal opening during short-term shade periods. Rapid stomatal closure during shade conserves water, but may also reduce CO₂ uptake. Because periods of alternating sun and shade can reduce accumulative water stress that would otherwise severely curtail carbon gain, some herbs are restricted to habitats with intermittent periods of shade. In contrast to herbaceous growth forms, woody species maintain relatively constant stomatal opening during both sun and shade periods. This results in greater CO₂ uptake, but with greater water loss. These two generalized response patterns for woody and herbaceous species to natural variations in sunlight conflict with conventional ideas of water use and carbon gain based on measurements made under constant light.     

Parasitofauna of freshwater fishes in the Serbian open waters: a checklist of parasites of freshwater fishes in Serbian open waters

As a result of freshwater fish parasitofauna investigations, throughout the past 75 years, the number of parasitic species is presented. This paper reviews the history, current state, and tendencies of the fish parasitofauna studies in Serbian open waters. Up to now, in total 170 parasitic species have been reported. Systematic parasitofauna investigations have been carried out on 54 freshwater fish species in waters of the Danube River Basin in Serbia, also in hill-mountanious watercourses, lakes and salmonid fishpond of Sjenicko-Pesterska plateau (south-west region of Serbia), Homolje area (east Serbia), and in Vlasina lake (south-east Serbia). This data review pointed that it is necessary to continue the investigation on fish parasites in Serbia. The data on freshwater fish parasites are important for evaluation of general influence on the community structure.     

Direct and indirect responses of a freshwater food web to a potent synthetic oestrogen

Endocrine-disrupting chemicals (EDCs) in municipal effluents directly affect the sexual development and reproductive success of fishes, but indirect effects on invertebrate prey or fish predators through reduced predation or prey availability, respectively, are unknown. At the Experimental Lakes Area in northwestern Ontario, Canada, a long-term, whole-lake experiment was conducted using a before-after-control-impact design to determine both direct and indirect effects of the synthetic oestrogen used in the birth control pill, 17α-ethynyloestradiol (EE2). Algal, microbial, zooplankton and benthic invertebrate communities showed no declines in abundance during three summers of EE2 additions (5–6 ng l⁻¹), indicating no direct toxic effects. Recruitment of fathead minnow (Pimephales promelas) failed, leading to a near-extirpation of this species both 2 years during (young-of-year, YOY) and 2 years following (adults and YOY) EE2 additions. Body condition of male lake trout (Salvelinus namaycush) and male and female white sucker (Catostomus commersonii) declined before changes in prey abundance, suggesting direct effects of EE2 on this endpoint. Evidence of indirect effects of EE2 was also observed. Increases in zooplankton, Chaoborus, and emerging insects were observed after 2 or 3 years of EE2 additions, strongly suggesting indirect effects mediated through the reduced abundance of several small-bodied fishes. Biomass of top predator lake trout declined by 23–42% during and after EE2 additions, most probably an indirect effect from the loss of its prey species, the fathead minnow and slimy sculpin (Cottus cognatus). Our results demonstrate that small-scale studies focusing solely on direct effects are likely to underestimate the true environmental impacts of oestrogens in municipal wastewaters and provide further evidence of the value of whole-ecosystem experiments for understanding indirect effects of EDCs and other aquatic stressors.